Nuclear magnetic resonance (NMR) detection is a widely-used technique for the detection of certain materials. For example, in the field of medicine, NMR imaging is used for, among other things, the early detection of tumors. Additionally, the phenomenon of NMR may be used in certain tomographic applications, such as the detection and measurement of the salinity, moisture, and hydrocarbon content of materials that are present in, for example, earth and rock samples, silo grains, or other enclosed structures.
The scientific principle underlying the various NMR applications is relatively simple. As is well known, atomic nuclei will precess at a determinable resonant frequency about an external magnetic field when the field is placed in the presence of the nuclei. When atomic nuclei precess about an external field, they induce in turn their own relatively weak nuclear magnetic fields. It happens that under certain conditions, the individual induced nuclear magnetic fields are additive. Consequently, the relative concentration of a target element nuclei within the magnetic field may be determined by measuring the combined magnetic moment generated by the resonantly rotating nuclei of the target element.
The measuring of the resulting combined magnetic moment of the target nuclei may be accomplished in a number of ways. One common method is to use an electrically conducting coil to measure the magnetic field which is generated by the target nuclei. Then, in accordance with Maxwell's laws, a voltage is induced in the coil by the constantly rotating magnetic moment generated by the precessing target nuclei. This voltage may be measured, and the magnitude of the voltage correlated to the number of nuclei (and, hence, relative concentration of the target element) that are within the uniform magnetic field.
This latter method of using the NMR of nuclei to measure the concentration of certain elements within a target zone generally requires that two conditions be met. The first condition, which is common to all NMR applications, is that the known magnetic field about which the nuclei precess needs to be of substantially uniform magnitude throughout the target zone. Moreover, to optimize signal strength, it is desireable that this generated field be as strong as possible, and, for certain applications, that the target zone be as large as possible. Secondly, again for the purpose of maximizing signal strength, a means to initially induce the target nuclei which are within the zone to precess about the uniform field in phase with each other should be provided. This is so because when the nuclei precess in phase, the magnitudes of the very small, individual nuclear moments which are generated by the nuclei will be additive. Stated differently, if these nuclear moments are out of phase, the resultant combined nuclear magnetic moments will be diminished. On the other hand, when the target nuclei precess in phase, a relatively large signal is generated from which the concentration of the target element within the uniform field zone may be determined.
Many present NMR systems which are used for measuring the content of certain elements in a given material are able to satisfy the above requirements. Unfortunately, because of the nature of the magnetic fields which are generated by typical field sources, such as permanent magnets, these NMR systems are typically able to achieve a field of constant strength only within the magnet. Thus, when a permanent magnet system is used as the field source for NMR applications, it is generally a requirement that the material which is to be examined be placed within the annulus of the magnet system to facilitate NMR tomography of the material. Obviously, in the case of silo grains, earth, etc., it is desirable that a uniform field be generated that is external to the magnet to permit in situ tomography of the material.
The present invention recognizes a need for providing an NMR device which can generate a uniform magnetic field external to the device for tomogaphy of earth, silo grains, and other materials. It is therefore an object of the present invention to provide an NMR device which can establish a substantially uniform magnetic field external to the device. It is a further object of the present invention to provide an NMR device which substantially maximizes the generated signal strength of the target nuclei. Yet another object of the present invention is to provide an NMR device which is easy to use and cost effective to manufacture.